The separation of carbon dioxide (CO2) from a mixed-gas source may be accomplished by a capture and regeneration process. More specifically, the process generally includes a selective capture of CO2, by, for example, contacting a mixed-gas source with a solid or liquid adsorber/absorber followed by a generation or desorption of CO2 from the adsorber/absorber. One technique describes the use of bipolar membrane electrodialysis for CO2 extraction/removal from solutions that contain dissolved inorganic carbon (DIC), primarily in the form of carbonate and bicarbonate ions, including seawater, brackish water, desalination brine and sodium/potassium bicarbonate or carbonate solutions.
For capture/regeneration systems, a volume of gas that is processed is generally inversely elated to a concentration of CO2 in the mixed-gas source, adding significant challenges to the separation of CO2 from dilute sources such as the atmosphere. CO2 in the atmosphere, however, establishes equilibrium with the total dissolved inorganic carbon (DIC) in the oceans, which is largely in the form of bicarbonate ions (HCO3−) at an ocean pH of 8.1-8.3. Therefore, a method for extracting CO2 from the DIC of the oceans would effectively enable the separation of CO2 from atmosphere without the need to process large volumes of air.
A method for extraction of the CO2 from seawater involves conversion of the DIC in seawater (primarily in the form of bicarbonate ion in seawater at its typical pH of 8.3) to dissolved CO2 gas via acidification of the seawater to a pH below 6. Even when all the DIC is converted to CO2 gas, typical DIC concentrations of 2.1 mmol in seawater correspond to an equilibrium partial pressure of the dissolved CO2 gas (assuming all the DIC has been converted to dissolved CO2 gas) of around 0.06 atmospheres (atm) (about 59 mBar) using a Henry's constant for CO2 of about 28.2 atm/M for 20° C. seawater. Assuming that the CO2 gas is dissolved in seawater (total dissolved solids (TDS) of 35,000 mg/L), and using a vapor pressure for 20° C. seawater of 0.023 atm (23 mbar), then an absolute pressure of 0.08 atm (81.8 mbar) must be produced above the seawater solution in order to reach the equilibrium partial pressure for CO2 and begin extracting CO2 gas. As CO2 gas is removed from solution, a concentration of the gas (CO2) decreases and therefore a CO2 equilibrium partial pressure decreases as the extraction proceeds. In order to extract a significant fraction of the dissolved gas, pressures significantly below this value must be achieved. Even if reverse osmosis brine produced in a desalination process is used rather than seawater, the DIC is typically increased by a factor of two, corresponding to an equilibrium partial pressure of 0.14 atm (about 142 mbar). Therefore, in order to extract the dissolved CO2 gas from the acidified seawater, the seawater must be flowed through some sort of degasification device, such as a membrane contactor of a desorption unit. The contactor works by allowing high surface area contact between the solution to be degassed and either a sweep gas or a vacuum with a partial pressure lower than the equilibrium partial pressure of the gas to be extracted. If the CO2 is to be used in subsequent fuel synthesis, fairly pure CO2 is required, eliminating the possibility of using nitrogen or argon sweep gases. Not only does vacuum require substantial energy, but the use of vacuum as a sweep gas also limits the lowest possible pressure to the vapor pressure of water (about 23 mbar (0.023 atm)) in that solution. One drawback of using vacuum can be that as the pressure is lowered to extract a larger fraction of the dissolved CO2, the fraction of water vapor in the extracted gas stream increases.